Exhibit #2: Uniform Variables

What We Did

• There are three ways you can pass information to a GLSL program.
  • Vertex attributes (covered)
  • Uniform variables (this exhibit)
  • Textures (wait 4 more weeks)
• Uniform variables are like global variables in a GLSL program.
  • You can declare one in the vertex shader, and another with the same name in the fragment shader. These two are actually the same variable.
• Their values are constant as the GLSL program runs; they can only be changed by the CPU (Javascript code).
• Declare one outside the scope of any function: uniform <type> <name>;

  uniform float amplitude;
  uniform float period;
  uniform float phase;
  uniform vec3 color;

• To set a uniform's value:
  • First, use your program.
  • Get the uniform location with gl.getUniformLocation(<program>, <uniform-name>).
  • Use gl.uniform[1234][fi](<location>, ...) to set the uniform value.
    • float uniform --> gl.uniform1f(<location>, <value>)
    • vec2 uniform --> gl.uniform2f(<location>, <value-1>, <value-2>)
    • vec3 uniform --> gl.uniform3f(<location>, <value-1>, <value-2>, <value-3>)
    • vec4 uniform --> gl.uniform4f(<location>, <value-1>, <value-2>, <value-3>, <value-4>)
    • int uniform --> gl.uniform1i(<location>, <value>)
    • And so on...
• Set all uniforms before you draw anything. Otherwise, their values are not updated when drawing.
• Uniforms can have matrix types, but we will talk about them in 2 weeks.